1000% protonmail.
Tutanota doesn't support encryption to anything outside of their own network which is why they have the "password protection" option which forces the receiver to open the e-mail on tutanota's server to read it. They also regularly delete accounts after a period of inactivity, so if you're not using it often you could find your account deleted. Tutanota users may as well be using gmail/hotmail IMO even though it's a bit more secure.
Protonmail is open source, free, and extremely secure. It also supports PGP so you can have end to end encryption to ANYBODY not just other protonmail users. As far as free providers go, there is no other option.